Spindle-shaped magnetic iron based alloy particles and process for producing the same

ABSTRACT

Disclosed herein are spindle-shaped magnetic iron based alloy particles containing at least one selected from the group consisting of Ni, Al, Si, P, Co, Mg, B and Zn, which have a particle length of 0.05 to 0.40  mu m, a crystallite size of 110 to 180  ANGSTROM , a specific surface area of 30 to 60 m2/g, a coercive force of 1,300 to 1,700 Oe and a saturation magnetization ( sigma s) of not less than 100 emu/g and a process for producing the same.

This is a divisional of application Ser. No. 08/118,287, filed Sep. 9,1993, now U.S. Pat. No. 5,466,308 which is an FWC of application Ser.No. 07/712,882 filed Jun. 11, 1991, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to spindle-shaped magnetic iron basedalloy particles for high-density recording, which have high outputcharacteristics and a low noise level, and a process for producing thesame.

Miniaturization, weight reduction and recording-time prolongation ofvideo or audio magnetic recording and reproducing apparatuses haverecently shown a remarkable progress. Especially, regarding video taperecorders (VTR) which have rapidly spread wide, the development ofsmaller-sized and lighter-weight VTR's for longer-time recording havebeen rapid. With this progress, magnetic recording media such as amagnetic tape have been strongly required to have a higher performanceand a higher recording density.

In other words, magnetic recording media are required to have higheroutput and lower the noise level. For this purpose, it is necessary toimprove the residual magnetic flux density (Br), the coercive force, thesurface smoothness of the magnetic media and the S/N ratio.

These characteristics of magnetic recording media have close relation tothe magnetic particles used for the magnetic recording media. In recentyears, magnetic iron based alloy particles have attracted attention dueto their coercive force and saturation magnetization which are superiorto those of conventional magnetic iron oxide particles, and have beenput to practical use as magnetic media such as digital audio tapes(DAT), 8-mm video tapes, Hi-8 tapes and video floppies. Such magneticiron based alloy particles, however, are also strongly demanded toimprove the characteristics.

The relationship between various characteristics of magnetic recordingmedia and the properties of magnetic particles used therefor will bedescribed in the following.

In order to obtain a higher recording performance, magnetic recordingmedia for VTR's are required to improve (1) the video S/N ratio, (2) thechroma S/N ratio and (3) the video frequency characteristics, as isobvious from the description in NIKKEI ELECTRONICS, May 3, pp. 82 to 105(1976).

In order to improve the video S/N ratio, it is important to make themagnetic particles smaller, and to improve the dispersibility of themagnetic particles in a vehicle, the orientation and the loadings of themagnetic particles in a coating film and the surface smoothness of themagnetic recording media.

It is known that a method of lowering the noise level of a magneticrecording medium by reducing the particle size of the magnetic particlesused therefore is effective as a method for improving the video S/Nratio.

The particle size of magnetic particles is often expressed by the valueof the specific surface area of the particles. It is generally knownthat the noise level of a magnetic recording medium has a tendency tolower with the increase in the specific surface area of the magneticparticles used.

This phenomenon is shown in, for example, FIG. 1 in Japanese PatentLaid-Open. No. 58-159231. The FIG. 1 shows the relationship between thespecific surface area of magnetic metal particles and the noise level ofthe magnetic tape produced therefrom. With the increase in the specificsurface area of the particles, the noise level lowers linearly.

Therefore, the magnetic particles are required to have a large specificsurface area in order to lower the noise level and improve the video S/Nratio.

However, if the specific surface area of the magnetic particles becomestoo large, it becomes more difficult to disperse the magnetic particlesin a vehicle (because the amount of binder per unit surface area of themagnetic particles is reduced) and to improve the orientation and theloadings thereof in the coating film, thereby making it impossible toobtain a good surface smoothness, and consequently it leads to thedeterioration of the video S/N ratio. Generally, the increase of thespecific surface area solely is rather unfavorable. It is thereforeimportant to select the optimum range of a specific surface area inconsideration of the technique of dispersing the magnetic particles in avehicle.

Regarding the relationship between the magnetic metal particles and thenoise, it is known that the crystallite size of the magnetic metalparticles has a relation to the noise.

This phenomenon is shown in, for example, FIG. 38 on page 123 of theCOLLECTED DATA ON MAGNETIC RECORDING MEDIA, Aug. 15 (1985), published bySynthetic Electronics Research. The FIG. 38 shows the relationshipbetween the crystallite size of the magnetic iron based alloy particlesand the noise of the magnetic tape produced therefrom. It is observedfrom FIG. 38 that with the reduction in the crystallite size of theparticles, the noise level is lowered.

It is therefore effective for lowering the noise level of a magneticrecording medium to reduce the crystallite size of the magnetic metalparticles as much as possible.

As described above, in order to improve the video S/N ratio and lowerthe noise level, magnetic particles which are excellent in thedispersibility in a vehicle, and the orientation and the loadings in acoating film, have a small crystallite size, an appropriate range (inparticular, about 30 to 60 m² /g) of a specific surface area and auniform particle size distribution, and contain no dendrites arerequired.

In order to improve the chroma S/N, it is necessary to improve thesurface property and the squareness ratio of the magnetic recordingmedium. For this purpose, magnetic particles having good dispersibilityand orientation property are useful. Such magnetic particles arerequired to have a large aspect ratio (major axial diameter/minor axialdiameter), an uniform particle size distribution and an appropriaterange of a specific surface area, and to contain no dendrites.

Furthermore, in order to improve the video frequency characteristics, itis necessary that the magnetic recording medium has a high coerciveforce (Hc) and a high remanense (Br).

In order to enhance the remanence (Br) of magnetic media, a highcoercive force (Hc) is required. It is important for the magneticparticles used for recording media such as video floppies, DAT's, 8-mmvideo tapes and Hi-8 tapes to have a coercive force of about 1,300 to1,700 Oe at present.

Since the coercive force of magnetic particles are generally caused bythe shape anisotropy, the coercive force has a tendency to increase withthe increase in the aspect ratio (major axial diameter/minor axialdiameter). On the other hand, the coercive force has a tendency toreduce with the reduction in the crystallite size. Therefore, if thecrystallite size is reduced in order to lower the noise level and toimprove the video S/N ratio, the coercive force is lowered and it isdifficult to improve the video frequency characteristics. Accordingly,the reduction in the small crystallite size while keeping the coerciveforce as high as possible is required.

Magnetic particles having a large saturation magnetization (σs) arenecessary for enhancing the residual magnetic flux density (Br) of themagnetic medium, and the residual magnetic flux density (Br) dependsupon the dispersibility of the magnetic particles in a vehicle and theorientation and the loadings of the magnetic particles in a coatingfilm.

Although magnetic iron based alloy particles have a larger saturationmagnetization than iron oxide magnetic particles, since they are veryfine particles having a particle size of not more than 1 μm, the surfaceactivity of the particles is so large that they react with oxygen eventhey are taken into air after forming an oxide film on the particlesurface by the surface oxidation after reduction, resulting in the greatdeterioration of the magnetic properties, in particular, the saturationmagnetization. The deficiency of oxidative stability causes, with thetime, the deterioration of the saturation magnetic flux density (Bm) andthe residual magnetic flux density (Br) of the magnetic recording mediumeven after the magnetic particles are coated with some binder as amagnetic coating. The saturation magnetization tends to be lowered to agreater extent as the magnetic iron based alloy particles become finer.Therefore, with the recent pronounced tendency to small magneticparticles, a balance in a large saturation magnetization and oxidizationstability becomes to be very important. Thus, the method for the surfaceoxidization of magnetic iron based alloy particles after reduction is animportant problem.

Magnetic iron based alloy particles are generally obtained byheat-treating in a reducing gas goethite particles as the startingmaterial, hematite particles obtained by dehydrating the goethiteparticles at a high temperature, the goethite particles containingmetals other than iron or the hematite particles containing metals otherthan iron.

As a method of producing goethite particles as the starting material,there are known a method(i) of producing acicular goethite particlescomprising adding not less than an equivalent of an alkali hydroxidesolution to an aqueous ferrous salt solution to obtain a suspensioncontaining ferrous hydroxide particles and carrying out oxidization bypassing an oxygen-containing gas into the suspension at a temperature ofnot higher than 80° C. and at a pH of not less than 11; and a method(ii)of producing spindle-shaped goethite particles comprising reacting anaqueous ferrous salt solution with an aqueous alkali carbonate solutionor a mixture of an aqueous alkali carbonate solution and an aqueousalkali hydroxide solution to obtain a suspension containing FeCO₃ or anFe-containing precipitate and carrying out oxidization by passing anoxygen-containing gas into the suspension.

The acicular goethite particles obtained by the method(i) has a largeaspect ratio (major axial diameter/minor axial diameter=not less than10) but contains dendrites and cannot be said to have an uniformparticle size distribution. And the magnetic iron based alloy particlesobtained by reduction of these acicular goethite particles have a highcoercive force due to the large aspect ratio (major axial diameter/minoraxial diameter), but contain dendrites and cannot be said to have anuniform particle size distribution.

On the other hand, the spindle-shaped goethite particles obtained by themethod(ii) have an uniform particle size distribution and do not containany dendrites, but it is difficult to prepare spindle-shaped goethiteparticles having a large aspect ratio (major axial diameter/minor axialdiameter). The preparation becomes more difficult as the particle lengthproduced becomes smaller. The magnetic iron based alloy particlesobtained by reduction of these spindle-shaped goethite particles have anuniform particle size distribution and contain no dendrites, so that thedispersibility in a vehicle and the orientation and the loadings thereofin a coating film are excellent, but since the aspect ratio (major axialdiameter/minor axial diameter) is small, and as a result it is difficultto obtain particles having a high coercive force.

A method of producing spindle-shaped goethite particles comprisingreacting an aqueous alkali carbonate solution with an aqueous ferroussalt solution to obtain a suspension containing FeCO₃ and passing anoxygen-containing gas into the suspension in the presence of acarboxylic acid such as citric acid and tartaric acid and a salt thereofis disclosed in Japanese Patent Application Laid-Open (KOKAI) No.50-80999. In this case, the goethite particles obtained have a smallaspect ratio (major axial diameter/minor axial diameter), as seen fromthe description of the specification: "Spheroidal particles close tospindle-shaped particles or spherical particles are obtained".

Various attempts have heretofore been conducted to increase the aspectratio (major axial diameter/minor axial diameter) of spindle-shapedgoethite particles so as to obtain magnetic iron based alloy particleswhich have an uniform particle size distribution without containing anydendrite and a high coercive force. For example, the methods aredescribed in Japanese Patent Application Laid-Open (KOKAI) Nos.59-232922 (1984), 60-21307 (1985) , 60-21819 (1985) , 60-36603 (1985) ,62-158801 (1987) and 2-51429 (1990). The spindle-shaped magnetic ironbased alloy particles which are obtained by these methods, however,disadvantageously have a large crystallite size.

Magnetic iron based alloy particles which have an uniform particle sizedistribution without containing any dendrite, a high coercive force, asmall crystallite size and an appropriate range of specific surface areahave been strongly demanded.

As a result of studies undertaken by the present inventors so as tosolve these problems, it has been found that spindle-shaped magneticiron based alloy particles which have a large aspect ratio (major axialdiameter/minor axial diameter), an uniform particle size distributionwithout any dendrite, a small crystallite size, an appropriate range ofspecific surface area, a high coercive force and a large saturationmagnetization can be obtained by aging a suspension containing FeCO₃ oran Fe-containing precipitate, passing an oxygen-containing gas into theaged suspension containing FeCO₃ or an Fe-containing precipitate in thepresence of propionic acid or salt thereof to obtain spindle-shapedgoethite particles, subjecting the thus-obtained spindle-shaped goethiteparticles to coating-treatment with some compound, and heat-treating thecoated particles in a reducing gas. The present invention has beenachieved on the basis of this finding.

SUMMARY OF THE INVENTION

In a first aspect of the present invention, there are providedspindle-shaped magnetic iron based alloy particles containing at leastone selected from the group consisting of Ni, Al, Si, P, Co, Mg, B andZn, which have a particle length (major axial diameter) of 0.05 to 0.40μm, a crystallite size of 110 to 180 Å, a specific surface area of 30 to60 m² /g, a coercive force of 1,300 to 1,700 Oe and a saturationmagnetization (σs) of not less than 100 emu/g.

In a second aspect of the present invention, there is provided a processfor producing spindle-shaped magnetic iron based alloy particlescomprising the steps of: adding an aqueous alkali carbonate solution ora mixture of an aqueous alkali carbonate solution and an aqueous alkalihydroxide solution to an aqueous ferrous salt solution so as to obtain asuspension containing FeCO₃ or an Fe-containing precipitate; aging thethus-obtained suspension containing FeCO₃ or an Fe-containingprecipitate; carrying out an oxidization by passing an oxygen-containinggas into the aged suspension containing FeCO₃ or an Fe-containingprecipitate in the presence of 0.1 to 10.0 mol % of propionic acid or asalt thereof based on Fe at 35° to 70° C. so as to obtain spindle-shapedgoethite particles; coating the thus-obtained spindle-shaped goethiteparticles with at least one compound selected from the group consistingof Ni, Al, Si, P, Co, Mg, B and Zn compounds; and heat-treating thecoated particles in a reducing gas.

In a third aspect of the present invention, there is provided a processfor producing spindle-shaped goethite particles comprising the steps of:adding an aqueous alkali carbonate solution or a mixture of an aqueousalkali carbonate solution and an aqueous alkali hydroxide solution to anaqueous ferrous salt solution so as to obtain a suspension containingFeCO₃ or an Fe-containing precipitate; aging the thus-obtainedsuspension containing FeCO₃ or an Fe-containing precipitate; andcarrying out oxidization by passing an oxygen-containing gas into theaged suspension containing FeCO₃ or an Fe-containing precipitate in thepresence of 0.1 to 10.0 mol % of propionic acid or a salt thereof basedon Fe at 35° to 70° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relationship between the addition of sodium propionateand the aspect ratio (major axial diameter/minor axial diameter) ofspindle-shaped goethite particles, wherein the curves A, B and Cindicate spindle-shaped goethite particles having particle length ofabout 0.3 to 0.5 μm, about 0.2 μm and about 0.1 μm, respectively.

FIG. 2 shows the relationship between the BET specific surface area andthe crystallite size of spindle-shaped magnetic iron based alloyparticles, wherein the marks Δ and × indicate the spindle-shapedmagnetic iron based alloy particles which are obtained by conventionalmethods, and the mark o indicates the magnetic iron based alloyparticles according to the present invention.

FIG. 3 shows the relationship between the coercive force and thecrystallite size of spindle-shaped magnetic iron based alloy particles,wherein the marks Δ and × indicate the spindle-shaped magnetic ironbased alloy particles which are obtained by conventional methods, andthe mark o indicates the magnetic iron based alloy particles accordingto the present invention.

FIGS. 4 to 6 are electron micrographs (×30000) of the spindle-shapedgoethite particles obtained in Example 1 and 5, and Comparative Example1, respectively.

FIGS. 7 to 10 are electron micrographs (×30000) of the spindle-shapedmagnetic iron based alloy particles obtained in Example 31, 35, 41 and46, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The aqueous ferrous salt solution used in the present invention is, forexample, an aqueous ferrous sulfate solution and an aqueous ferrouschloride solution.

As the aqueous alkali carbonate solution, aqueous solutions of sodiumcarbonate, potassium carbonate, ammonium carbonate etc. may be used, andas the aqueous alkali hydroxide solution, aqueous solutions of sodiumhydroxide, potassium hydroxide etc. may be used.

The preferred mixing ratio of the aqueous alkali solution (the aqueousalkali carbonate solution or a mixture of the aqueous alkali carbonatesolution and the aqueous alkali hydroxide solution) to the aqueousferrous salt solution is not less than 1.05/1, preferably 1.05/1 to4.0/1 in the molar ratio (such as 2Na/Fe) of alkali (expressed as sodiumcarbonate) based on Fe.

The suspension containing FeCO₃ or an Fe-containing precipitate is agedat 35° to 60° C. for 50 to 800 minutes in an inert atmosphere by passingan inert gas such as N₂ gas in the suspension. The suspension is stirredby bubbling with the gas or by a mechanical operation.

If the aging temperature is lower than 35° C., the aspect ratio (majoraxial diameter/minor axial diameter) becomes too small. Even if theaging temperature exceeds 60° C., it is possible to obtain thespindle-shaped goethite particles having a large aspect ratio (majoraxial diameter/minor axial diameter), but it is meaningless to raise theaging temperature to a higher degree than necessary.

If the aging time is less than 50 minutes, it is difficult to obtain thespindle-shaped goethite particles having a large aspect ratio (majoraxial diameter/minor axial diameter). Even if the aging time exceeds 800minutes, it is possible to obtain the spindle-shaped goethite particleshaving a large aspect ratio (major axial diameter/minor axial diameter),but it is meaningless to prolong the aging time more than necessary.

The pH of the suspension at the time of aging is 7 to 11. If the pH isless than 7 or more than 11, it is difficult to obtain thespindle-shaped goethite particles.

The reaction temperature for the oxidization for producing the goethiteparticles is 35° to 70° C. If the reaction temperature is lower than 35°C., it is difficult to obtain the spindle-shaped goethite particleshaving a large aspect ratio (major axial diameter/minor axial diameter).If it exceeds 70° C., granular hematite particles become mixed in thespindle-shaped goethite particles.

The oxidization for producing the goethite particles is carried out bypassing an oxygen-containing gas (e.g., air) into the suspension. Thesuspension is stirred by bubbling with the gas or by a mechanicaloperation. The amount of oxygen-containing gas passed into thesuspension and the oxidizing time are appropriately selected inaccordance with the scale of a reaction vessel and the amount ofsuspension treated.

It is necessary to add propionic acid or a salt thereof in the reactionliquid at a stage before passing an oxygen-containing gas thereinto foroxidization. For example, propionic acid or a salt thereof may be addedto the aqueous alkali carbonate solution, the mixture of the aqueousalkali carbonate solution and the aqueous alkali hydroxide solution, theferrous salt solution, or the suspension containing FeCO₃ or anFe-containing precipitate before passing an oxygen-containing gasthereinto for oxidization.

As the salt of propionic acid in the present invention, sodiumpropionate, potassium propionate, calcium propionate, zinc propionate,cobalt propionate, magnesium propionate etc. may be used.

The amount of propionic acid or the salt thereof is in the range of 0.1to 10.0 mol % based on Fe.

Addition of less than 0.1 mol % of propionic acid or the salt thereof isineffective for increasing the aspect ratio (major axial diameter/minoraxial diameter) and reducing the crystallite size. Although addition ofmore than 10.0 mol % of propionic acid and the salt thereof is alsoeffective for increasing the aspect ratio (major axial diameter/minoraxial diameter) and reducing the crystallite size, it is meaningless toadd propionic acid or the salt thereof more than necessary.

The spindle-shaped goethite particles obtained may be heated at 250° to350° C. to obtain spindle-shaped hematite particles.

It is preferable to coat the starting material (goethite particles orhematite particles) with at least one compound selected from the groupconsisting of compounds of Ni, Al, Si, P, Co, Mg, B and Zn in order tokeep the particle shape and to prevent sintering between particles.Examples of such a compound are acetates and nitrates of Ni, Al, Co, Mgand Zn, aluminic acid, boric acid, silicic acid and phosphoric acid.Since these compounds have not only a sintering preventive effect butalso an activity of controlling the reducing speed, it is preferable touse them in the form of a combination as occasion demands.

Especially, it is preferable to treat the starting material with atleast one selected from the group consisting of acetates of Ni, Al, Co,Mg and Zn and nitrates of Ni, Al, Co, Mg and Zn, and then treat it withat least one selected from the group consisting of boric acid, aluminicacid, silicic acid and phosphoric acid.

Thus obtained coated particles are reduced at a temperature range of300° to 550° C.

If the temperature exceeds 550° C., the reduction rapidly progresses,thereby deforming the particle shape and causing sintering betweenparticles.

If the temperature is lower than 300° C., the progress of the reductionis so slow that it takes a long time.

The flow rate of the reducing gas for the heat treatment and theheat-treating time are appropriately selected in accordance with thescale of a reaction vessel and the amount of particles treated.

As the reducing gas, H₂ gas may be used.

The magnetic iron based alloy particles after the heat treatment in thereducing gas can be taken out into air by a known method, for example,the method comprising immersing the magnetic iron based alloy particlesin an organic solvent such as toluene or the method comprising replacingthe atmosphere around the magnetic iron based alloy particles with aninert gas and then gradually increasing the oxygen content in the inertgas by introducing air until it becomes air so as to carry outoxidization gradually.

Before the heat-treatment in the reducing gas, it is preferable to heatthe starting material in a non-reducing atmosphere by a conventionalmethod prior to the reduction in order to control the magneticproperties, the powder properties and the shape of the particles.

The heat treatment in the non-reducing atmosphere may be carried out ata temperature range of 300° to 800° C. in a stream of air, oxygen gas ornitrogen gas.

If the temperature exceeds 800° C., the particle shape is deformed, andsintering between particles is caused.

Furthermore, it is preferable to stabilize the magnetic iron based alloyparticles after the heat treatment in the reducing gas by subjectingthem to the surface oxidization treatment at 30° to 200° C. in anoxygen-containing gas. The oxygen content in the oxygen-containing gasis preferably 0.02 to 20% by volume. If an oxygen-containing gas inwhich the oxygen content is less than 0.02% by volume is used, theprogress of the oxidization reaction is so slow that it unfavorablytakes a long time.

The oxygen-containing gas is ordinarily a mixed gas of air and an inertgas. As an example of the inert gas, N₂ gas may be used.

Although the treating temperature of lower than 30° C. produces noproblem, the progress of the oxidization reaction is so slow that itunfavorably takes a long time for treating. On the other hand, if itexceeds 200° C. and the mixing ratio is as described above, since theoxidization reaction progresses too rapidly, the magnetic properties, inparticular, the saturation magnetization (σs) unfavorably lowers.

The surface oxidization-treatment time is appropriately determined inaccordance with the treating temperature and the oxygen content in themixed gas.

Especially, the surface oxidization treatment is preferably carried outin multiple stages by subsequently increasing the oxygen content of theoxygen-containing gas each time the heat generation produced by theoxidization reaction by the gas reaches the peak, as seen from examplesdescribed later. The surface oxidization treatment is ordinarily carriedout at 30° to 200° C.

In order to improve various properties of magnetic iron based alloyparticles, metals other than Fe such as Co, Ni, Cr, Zn, Al, Mn and thelike which are ordinarily added when producing the goethite particles asthe starting material may be added. In this case, it is also possible toobtain the magnetic iron based alloy particles having a large aspectratio (major axial diameter/minor axial diameter), a small crystallitesize, an appropriate range of a specific surface area and a highcoercive force.

The magnetic iron based alloy particles obtained by the above-describedmethod are spindle-shaped and have the following physical properties.

The particle length (major axial diameter) is 0.05 to 0.40 μm,preferably 0.05 to 0.35 μm, and the aspect ratio (major axialdiameter/minor axial diameter) is preferably 6 to 20. The crystallitesize is 110 to 180 Å, preferably 120 to 180 Å, and the specific surfacearea is 30 to 60 m² /g, preferably 40 to 60 m² /g. The coercive force is1,300 to 1,700 Oe, preferably 1,400 to 1,700 Oe, and the saturationmagnetization (σs) is not less than 100 emu/g, preferably 105 to 140emu/g. The reduction ratio of the saturation magnetization of themagnetic iron based alloy particles which have been allowed to stand ata temperature of 40° C. and a relative humidity of 70% for 4 days is notmore than 12%, preferably not more than 8%. If the saturationmagnetization reduction ratio is not more than 12%, the saturationmagnetization ordinarily becomes 100 to 140 emu/g.

The particle length is a distance between one end and another end. Theparticle length is a diameter at the middle.

The magnetic iron based alloy particles may contain at least oneselected from the group consisting of Ni, Al, Si, P, Co, Mg, B and Zn,which is added to the magnetic iron based alloy particles at the time ofthe coating treatment or production of the goethite particles.

It is also possible to obtain more preferable magnetic iron based alloyparticles having a crystallite size, a coersive force and a specificsurface area which simultaneously satisfy the following relationships:

-8×[crystallite size (Å)]+[coercive force (Oe)]≧110, and

[crystallite size (Å)]+2×[specific surface area (m² /g)]≦290.

The reason why the spindle-shaped goethite particles having a largeaspect ratio are obtained in accordance with the present invention isconsidered to be based on a synergistic effect of the aging process andthe oxidization process carried out in the presence of propionic acid ora salt thereof from the fact that it is impossible to obtainspindle-shaped goethite particles having a large aspect ratio both inthe case of carrying out oxidization in the absence of propionic acid orthe salt thereof after the aging process, and in the case of omittingthe aging process and carrying out the oxidization in the presence ofpropionic acid or a salt thereof.

The reason why the magnetic iron based alloy particles which have anappropriate range of specific surface area and a high coercive force inspite of a small crystallite size are obtained is considered to be asfollows.

By the observant electron microscopic observation of the spindle-shapedgoethite particles produced by obtaining a suspension containing FeCO₃or an Fe-containing precipitate by adding an aqueous alkali carbonatesolution or a mixture of an aqueous alkali carbonate solution and anaqueous hydroxide solution to an aqueous ferrous salt solution andcarrying out oxidization by passing an oxygen-containing gas into thesuspension, crystals of the long and slender primary particles havegrown in the form of a bundle of straws. By a conventional method, sincea goethite particle greatly grows in the widthwise direction incompliance of the increase of the number of the primary particles,spindle-shaped particles having a small aspect ratio are apt to beobtained.

In addition, in the case of producing magnetic iron based alloyparticles by subjecting the spindle-shaped goethite particles to asintering preventive treatment and subjecting the resultant particles toheat treatment in a reducing gas by a conventional method, since thegrowth of the crystals between the long and slender primary particles inthe form of a bundle of straws progresses, the crystallite size of themagnetic iron based alloy particles obtained is inevitably larger thanthat of the magnetic iron based alloy particles produced from aciculargoethite particles as the starting material which are obtained by theoxidization of Fe(OH)₂.

In contrast, it is considered that in the spindle-shaped goethiteparticles in the present invention, since it is possible to reduce thegrowth of a particle in the widthwise direction due to the synergisticeffect of the aging process and the oxidization process in the presenceof propionic acid or a salt thereof, the aspect ratio thereof isincreased and the growth of the primary particles in the widthwisedirection during reduction is suppressed by controlling the growth ofthe goethite particles in the widthwise direction, and as a result, thecrystallite size is reduced.

FIG. 1 shows the relationship between the addition of sodium propionateand the aspect ratio of spindle-shaped goethite particles.

In FIG. 1, the goethite particles were obtained in the same way as inExamples 1, 5 and 7, which will be described later, except that 0 to 10mol % of sodium propionate based on Fe is present therein. The curves A,B and C indicate spindle-shaped goethite particles having particlelength of about 0.3 to 0.5 μm, about 0.2 μm and about 0.1 μm,respectively.

As is clear from FIG. 1, the aspect ratio of the spindle-shaped goethiteparticles has a tendency to increase with the increase of the content ofsodium propionate.

FIG. 2 shows the relationship between the BET specific surface area andthe crystallite size of spindle-shaped magnetic iron based alloyparticles.

In FIG. 2, the marks Δ and × indicate the spindle-shaped magnetic ironbased alloy particles which were obtained by conventional methodsdescribed in Japanese Patent Application Laid-Open (KOKAI) Nos. 60-36603and 2-51429, respectively, and the mark o indicates the spindle-shapedmagnetic iron based alloy particles according to the present invention.The spindle-shaped magnetic iron based alloy particles according to thepresent invention have an appropriate specific surface area in spite ofthe smaller crystallite size than that of the spindle-shaped magneticiron based alloy particles which were obtained by a conventional method.

FIG. 3 shows the relationship between the coercive force and thecrystallite size of spindle-shaped magnetic iron based alloy particles.

In FIG. 3, the marks Δ and × indicate the spindle-shaped magnetic ironbased alloy particles which were obtained by conventional methodsdescribed in Japanese Patent Application Laid-Open (KOKAI) Nos. 60-36603and 2-51429, respectively, and the mark o indicates the spindle-shapedmagnetic iron bsed alloy particles according to the present invention.The spindle-shaped magnetic iron based alloy particles according to thepresent invention have a high coercive force in spite of the smallercrystallite size than that of the spindle-shaped magnetic iron basedalloy particles which were obtained by a conventional method.

Generally, the crystallite size, the specific surface area etc. arechangeable depending upon the environmental conditions, the particlesize of the starting material and the like. As is clear from FIGS. 2 and3, it is difficult to balance such properties with the environmentalconditions in the case of using the spindle-shaped magnetic iron bsedalloy particles obtained by a conventional method as the startingmaterial.

Since the spindle-shaped magnetic iron based alloy particles accordingto the present invention have a large aspect ratio (major axialdiameter/minor axial diameter), an uniform particle size distributionwithout any dendrite, a small crystallite size, an appropriate range ofa specific surface area, a high coercive force and a large saturationmagnetization, and they are excellent in oxidative stability when theyare subjected to surface oxidization treatment, they are suitable as themagnetic particles for high-density recording at a low noise level.

Furthermore, if the spindle-shaped magnetic iron based alloy particlesaccording to the present invention are used for producing a magnetictape or magnetic disc, the dispersibility in a vehicle is good and it ispossible to obtain a good magnetic recording medium having an excellentrecording performance and good signal to noise S/N ratio.

EXAMPLES

The present invention will now be explained with reference to thefollowing examples and comparative examples. It is to be understood,however, that the present invention is not restricted by these examples.

(1) The particle length and the aspect ratio (major axial diameter/minoraxial diameter) in the following examples and comparative examples areexpressed by the average of the values measured from electronmicrographs, and the specific surface area is expressed by the valuemeasured from the N₂ gas adsorption by a BET method.

(2) The crystallite size is expressed by the diameter of the crystal inthe direction perpendicular to the plane measured by X-ray diffraction.The value is calculated from the measured line profile of thediffraction pattern by using the following Scherrer's formula. ##EQU1##wherein β: the true half-width of the diffraction peak with the width ofthe machine subtracted therefrom

K: Scherrer constant (0.9)

λ: the wavelength of X-ray

Θ: Brugg angle

(3) The oxidative stability is expressed by the percentage of thesaturation magnetization reduction of the particles which was allowed tostand at 40° C., 70% RH for 4 days.

Reduction ratio of the saturation magnetization (%) ##EQU2## <Productionof Spindle-Shaped Goethite Particles>

Example 1

600 l of the aqueous solution of 1.35 mol/l of Na₂ CO₃ containing 1945 g(equivalent to 5.0 mol % based on Fe) of sodium propionate was chargedin a reaction vessel which was maintained in a non-oxidizing atmosphereby passing N₂ gas thereinto at a rate of 3.4 cm/sec. With this aqueoussolution was mixed 300 l of an aqueous ferrous sulfate solutioncontaining 1.35 mol/l of Fe²⁺ at a temperature of 50° C. to produceFeCO₃.

The suspension containing FeCO₃ was held at 50° C. for 300 minutes whilecontinuously blowing N₂ gas thereinto at a rate of 3.4 cm/sec.Thereafter, air was passed into the suspension containing FeCO₃ at 50°C. at a rate of 2.8 cm/sec for 5.5 hours to produce yellowish brownprecipitated particles. The pH of the suspension during the aeration was8.5 to 9.5.

The yellowish brown precipitated particles were filtered out, washedwith water, dried and pulverized by an ordinary method.

The thus-obtained yellowish brown particles were proved to be goethiteparticles as a result of X-ray diffraction. As is obvious from theelectron microphotograph (×30000) shown in FIG. 4, they werespindle-shaped particles having an average particle length of 0.31 μmand an aspect ratio (major axial diameter/minor axial diameter) of15.8/1 with a uniform particle size distribution and containing nodendrites.

Examples 2 to 7, Comparative Examples 1 to 3

Spindle-shaped goethite particles were obtained in the same way as inExample 1 except for varying the kind, the concentration and the amountof aqueous alkali carbonate solution used, the kind, the amount and theadding time of propionic acid or a salt thereof, the kind, theconcentration and the amount of aqueous ferrous salt solution used, thetemperature of the suspension, a addition of an aqueous alkali hydroxidesolution in the process of producing the suspension containing anFe-containing precipitate, the temperature and the time at the agingprocess, and the temperature and the reaction time at the oxidizationprocess.

The main producing conditions and various properties of the particlesobtained are shown in Tables 1 and 2.

Any of the spindle-shaped goethite particles obtained in Examples 2 to 7had a uniform particle size distribution and contained no dendrites.

The electron micrograph (×30000) of the spindle-shaped goethiteparticles obtained in Example 5 is shown in FIG. 5.

The spindle-shaped goethite particles obtained in Comparative Example 1had a large particle length and a small aspect ratio (major axialdiameter/minor axial diameter), as shown in the electron micrograph(×30000) of FIG. 6.

Example 8

A precipitate of Co(OH)₂ was produced by adding 3.65 of a solution of10.0 mol/l of NaOH to 9.1 l of the aqueous solution of 2.0 mol/l ofCoSO₄ ·7H₂ O. After discharging as much supernatant of the Co(OH)₂precipitate as possible, 36.5 mol of propionic acid was added to theprecipitate to prepare 25 l of a cobalt propionate solution in totalvolume.

600 l of the aqueous solution of 1.35 mol/l of Na₂ CO₃ was charged in areaction vessel which was maintained in a non-oxidizing atmosphere bypassing N₂ gas thereinto at a rate of 3.4 cm/sec. With this aqueoussolution was mixed 300 l of an aqueous ferrous sulfate solutioncontaining 1.35 mol/l of Fe²⁺ at a temperature of 48° C. to produceFeCO₃.

The cobalt propionate solution prepared in advance was added to thesuspension containing FeCO₃.

The suspension containing FeCO₃ was held at 48° C. for 300 minutes whilecontinuously blowing N₂ gas thereinto at a rate of 3.4 cm/sec.Thereafter, air was passed into the suspension containing FeCO₃ at 48°C. at a rate of 2.8 cm/sec. for 5.1 hours to produce yellowish brownprecipitated particles. The pH of the suspension during the aeration was8.4 to 9.5.

The yellowish brown precipitated particles were filtered out, washedwith water, dried and pulverized by an ordinary method. The mainproducing conditions and various properties of the particles obtainedare shown in Tables 1 and 2.

The thus-obtained yellowish brown particles were proved to be goethiteparticles as a result of X-ray diffraction. They were spindle-shapedparticles having an average particle length of 0.27 μm and an aspectratio (major axial diameter/minor axial diameter) of 14.8/1 with auniform particle size distribution and containing no dendrites.

Example 9

Spindle-shaped goethite particles were obtained in the same way as inExample 8 except for using 3.0 mol/l of zinc propionate in place of 4.5mol/l of cobalt propionate.

The main producing conditions and various properties of the particlesobtained are shown in Tables 1 and 2.

The spindle-shaped goethite particles obtained in Example 9 had auniform particle size distribution and contained no dendrites.

<Production of Spindle-Shaped Hematite Particles>

Example 10 and 11

Spindle-shaped hematite particles were obtained by dehydrating thespindle-shaped goethite particles obtained in Examples 2 and 5 at 300°C. in air.

The hematite particles obtained had an average particle length of 0.36μm and an aspect ratio (major axial diameter/minor axial diameter) of15.0/1; and an average particle length of 0.18 μm and an aspect ratio(major axial diameter/minor axial diameter) of 11.0/1, respectively,according to the observation through an electron microscope.

                                      TABLE 1                                     __________________________________________________________________________           Production of FeCO.sub.3 or Fe-containing precipitate                  Examples                                                                             Aqueous alkali Aqueous alkali                                          and    carbonate solution                                                                           hydroxide solution                                                                              Propionic acid or a salt              Compar-     Concen-        Concen-      thereof                               ative       tration                                                                            Amount    tration                                                                            Amount                                                                             2Na/     Amount                                                                             Time for                   example                                                                              Kind (mol/l)                                                                            (l)  Kind (mol/l)                                                                            (l)  Fe Kind  (mol %)                                                                            addition                   __________________________________________________________________________    Example 1                                                                            Na.sub.2 CO.sub.3                                                                  1.35 600  --   --   --   2.0                                                                              Sodium                                                                              5.0  A*1                                                                propionate                            Example 2                                                                            Na.sub.2 CO.sub.3                                                                  1.16 420  NaOH 1.35 180  1.5                                                                              Sodium                                                                              5.0  B*2                                                                propionate                            Example 3                                                                            K.sub.2 CO.sub.3                                                                    0.675                                                                             800  --   --   --   2.0                                                                              Potassium                                                                           7.0  D*4                                                                propionate                            Example 4                                                                            Na.sub.2 CO.sub.3                                                                  1.35 600  --   --   --   2.0                                                                              Propionic                                                                           3.0  C*3                                                                acid                                  Example 5                                                                            Na.sub.2 CO.sub.3                                                                  1.20 600  --   --   --   2.0                                                                              Sodium                                                                              4.0  A*1                                                                propionate                            Example 6                                                                            K.sub.2 CO.sub.3                                                                   1.20 600  --   --   --   2.0                                                                              Sodium                                                                              3.0  A*1                                                                propionate                            Example 7                                                                            Na.sub.2 CO.sub.3                                                                  1.10 600  --   --   --   2.0                                                                              Sodium                                                                              3.0  A*1                                                                propionate                            Example 8                                                                            Na.sub.2 CO.sub.3                                                                  1.35 600  --   --   --   2.0                                                                              Cobalt                                                                              4.5  D*4                                                                propionate                            Example 9                                                                            Na.sub.2 CO.sub.3                                                                  1.35 600  --   --   --   2.0                                                                              Zinc  3.0  D*4                                                                propionate                            Comparative                                                                          Na.sub.2 CO.sub.3                                                                  1.35 600  --   --   --   2.0                                                                              --    --   --                         example 1                                                                     Comparative                                                                          Na.sub.2 CO.sub.3                                                                  1.35 600  --   --   --   2.0                                                                              Sodium                                                                              5.0  A*1                        example 2                               propionate                            Comparative                                                                          Na.sub.2 CO.sub.3                                                                  1.35 600  --   --   --   2.0                                                                              --    --   --                         example 3                                                                     __________________________________________________________________________                                        Production of FeCO.sub.3                                                      or Fe-containing precipitate                                           Examples                                                                             Aqueous ferrous salt                                                   and    solution                                                               Compar-     Concen-   Temper-                                                 ative       tration                                                                            Amount                                                                             ature                                                   example                                                                              Kind (mol/l)                                                                            (l)  (°C.)               __________________________________________________________________________                                 Example 1                                                                            FeSO.sub.4                                                                         1.35 300  50                                                      Example 2                                                                            FeSO.sub.4                                                                         1.35 300  50                                                      Example 3                                                                            FeCl.sub.2                                                                         1.35 200  42                                                      Example 4                                                                            FeSO.sub.4                                                                         1.35 300  55                                                      Example 5                                                                            FeSO.sub.4                                                                         1.20 300  45                                                      Example 6                                                                            FeCl.sub.2                                                                         1.20 300  40                                                      Example 7                                                                            FeSO.sub.4                                                                         1.10 300  40                                                      Example 8                                                                            FeSO.sub.4                                                                         1.35 300  48                                                      Example 9                                                                            FeSO.sub.4                                                                         1.35 300  48                                                      Comparative                                                                          FeSO.sub.4                                                                         1.35 300  50                                                      example 1                                                                     Comparative                                                                          FeSO.sub.4                                                                         1.35 300  50                                                      example 2                                                                     Comparative                                                                          FeSO.sub.4                                                                         1.35 300  50                                                      example 3                                        __________________________________________________________________________     (Note)                                                                        *1: A represents an addition of propionic acid or a salt thereof to an        aqueous alkali carbonate solution.                                            *2: B represents an addition of propionic acid or a salt thereof to an        aqueous alkali carbonate · alkali hydroxide solution.                *3: C represents an addition of propionic acid or a salt thereof to an        aqueous ferrous salt solution.                                                *4: D represents an addition propionic acid or a salt thereof to a            suspension containing FeCO.sub.3.                                        

                                      TABLE 2                                     __________________________________________________________________________                                      Spindle-shaped goethite                                                       particles                                                                          Aspect ratio                           Examples          Oxidization          (major axial                           and    Aging                 Reaction                                                                           Particle                                                                           diameter/                              Comparative                                                                          Temperature                                                                          Time    Temperature                                                                          time length                                                                             minor axial                            example                                                                              (°C.)                                                                         (min)                                                                             pH  (°C.)                                                                         (hour)                                                                             (μm)                                                                            diameter)                              __________________________________________________________________________    Example 1                                                                            50     300 8.5-9.5                                                                           50     5.5  0.31 15.8/1                                 Example 2                                                                            50      60 8.4-9.4                                                                           50     5.8  0.36 15.1/1                                 Example 3                                                                            43     360 8.4-9.3                                                                           43     4.1  0.23 15.1/1                                 Example 4                                                                            55     300 8.5-9.5                                                                           56     5.6  0.47 16.2/1                                 Example 5                                                                            45     150 8.5-9.4                                                                           40     5.0  0.18 11.4/1                                 Example 6                                                                            40     150 8.5-9.3                                                                           38     5.1  0.15 10.9/1                                 Example 7                                                                            40     100 8.4-9.3                                                                           36     5.0  0.10  8.8/1                                 Example 8                                                                            48     300 8.4-9.5                                                                           48     5.1  0.27 14.8/1                                 Example 9                                                                            48     300 8.4-9.5                                                                           48     5.4  0.35 16.5/1                                 Comparative                                                                          --     --  8.5-9.5                                                                           50     4.5  0.31  7.8/1                                 example 1                                                                     Comparative                                                                          --     --  8.5-9.5                                                                           50     4.5  0.30  7.5/1                                 example 2                                                                     Comparative                                                                          50     300 8.5-9.5                                                                           50     5.5  0.31 12.4/1                                 example 3                                                                     __________________________________________________________________________

<Coating of Spindle-Shaped Goethite Particles with Metal Compound>

Example 12

The goethite particles obtained in Example 1 were filtered out andwashed with water to obtain a presscake. 4,000 g of the thus-obtainedpresscake (equivalent to 1,000 g of the the goethite particles) wassuspended in 30 l of water. The pH of the suspension was 9.1.

120 g of Al (NO₃)₃ ·9H₂ O was added to the suspension so that it was12.0 wt % of the goethite particles, and the resultant mixture wasstirred for 10 minutes.

211 g (21.1 wt % based on the goethite particles) of Co(CH₃ COO)₂ ·4H₂ Owas then added to the suspension and the mixture was stirred for 10minutes. The pH of the suspension was 5.03.

A solution of 180 g (18.0 wt % based on the goethite particles) of H₃BO₃ was slowly added to the suspension and the suspension and themixture was stirred for 15 minutes.

After NaOH was further added to the suspension to adjust the pH to 9.5,the suspension was filtered by a filter press, washed with hot water anddried to obtain goethite particles coated with Al, Co and B compounds.

The contents of the elements Al, Co and B in the particles obtained were0.71 wt %, 4.24 wt % and 0.74 wt % based on the particles, respectively.

The main treating conditions are shown in Table 3.

Examples 13 to 20, Comparative Examples 4 to 6

Spindle-shaped goethite particles were coated in the same way as inExample 12 by varying the kind and amount of Al, Si, P, Ni, Mg, Co, B orZn compound added.

The main treating conditions are shown in Table 3.

In the Example 16, sodium aluminate were added to the suspension afterthe pH thereof was adjusted to the range of 7 to 9.

<Production of Spindle-shaped Hematite Particles Coated with MetalCompound>

Example 21

The spindle-shaped hematite particles were obtained by dehydrating thespindle-shaped hematite particles obtained in Example 13 at atemperature of 300° C. in air.

The obtained spindle-shaped hematite particles had an average particlelength of 0.36 μm and an aspect ratio (major axial diameter/minor axialdiameter) of 15.0/1 according to the observation through an electronmicroscope.

                  TABLE 3                                                         ______________________________________                                        Example and                                                                            Treated    Compound                                                  Comparative                                                                            particles                  Amount                                    example  (obtained in)                                                                            Kind            (wt %)                                    ______________________________________                                        Example 12                                                                             Example 1  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 12.0                                                          Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             21.1                                                          H.sub.3 BO.sub.3                                                                              18.0                                      Example 13                                                                             Example 2  Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             14.0                                                          Ni(CH.sub.3 COO).sub.2.4H.sub.2 O                                                              1.5                                                          Al(NO.sub.3).sub.3.9H.sub.2 O                                                                  8.0                                                          H.sub.3 BO.sub.3                                                                              14.0                                      Example 14                                                                             Example 3  Sodium           0.8                                                          hexametaphosphate                                                             Sodium silicate 13.5                                                          Sodium aluminate                                                                               7.0                                                          Co(NO.sub.3).sub.2.6H.sub.2 O                                                                  6.0                                      Example 15                                                                             Example 4  Co(NO.sub.3).sub.2.6H.sub.2 O                                                                 15.0                                                          Zn(CH.sub.3 COO).sub.2.2H.sub.2 O                                                              3.0                                                          H.sub.3 BO.sub.3                                                                              15.0                                      Example 16                                                                             Example 5  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 15.0                                                          Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             12.7                                                          H.sub.3 BO.sub.3                                                                              20.0                                      Example 17                                                                             Example 6  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 16.0                                                          Co(NO.sub.3).sub.2.6H.sub.2 O                                                                 11.0                                                          H.sub.3 BO.sub.3                                                                              16.0                                      Example 18                                                                             Example 7  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 18.0                                                          Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             16.9                                                          H.sub.3 BO.sub.3                                                                              17.0                                      Example 19                                                                             Example 8  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 13.0                                                          Mg(CH.sub.3 COO).sub.2.4H.sub.2 O                                                              6.5                                                          H.sub.3 BO.sub.3                                                                              18.0                                      Example 20                                                                             Example 9  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 11.0                                                          Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             16.9                                                          H.sub.3 BO.sub.3                                                                              20.0                                      Comparative                                                                            Comparative                                                                              Al(NO.sub.3).sub.3.9H.sub.2 O                                                                  9.0                                      example 4                                                                              example 1  Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             12.7                                                          H.sub.3 BO.sub.3                                                                              15.0                                      Comparative                                                                            Comparative                                                                              Al(NO.sub.3).sub.3.9H.sub.2 O                                                                  9.0                                      example 5                                                                              example 2  Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             12.7                                                          H.sub.2 BO.sub.3                                                                              15.0                                      Comparative                                                                            Comparative                                                                              Al(NO.sub.3).sub.3.9H.sub.2 O                                                                  9.0                                      example 6                                                                              example 3  (CH.sub.3 COO).sub.2.4H.sub.2 O                                                               12.7                                                          H.sub.3 BO.sub.3                                                                              15.0                                      ______________________________________                                    

Example 22

The goethite particles obtained in Example 1 were filtered out andwashed with water to obtain a presscake. The thus-obtained presscakeequivalent to 10 kg of the goethite particles was suspended in 200 l ofwater. The pH of the suspension was 9.2.

1.3 kg (13.0 wt % based on the goethite particles) of Al(NO₃)₃ ·9H₂ Owas added to the suspension. 2.32 kg (23.2 wt % based on the goethiteparticles) of Co(CH₃ COO)₂ ·4H₂ O was further added to the suspensionand the mixture was stirred for 15 minutes. The pH of the suspension was4.70.

After NaOH was added to the suspension to adjust the pH to 9.8, 150 g(1.5 wt % based on the goethite particles) of oleic acid was added.

After the suspension was thoroughly washed with hot water of 60° C. by arotary filter, 1.5 kg of H₃ BO₃ was added (15 wt % based on the goethiteparticles) and the mixture was stirred for 20 minutes.

The suspension was further filtered by a filter press and dried toobtain goethite particles coated with Al, Co and B compounds.

The contents of the elements Al, Co and B in the particles obtained were0.89 wt %, 5.32 wt % and 0.68wt % based on the particles, respectively.

The main treating conditions are shown in Table 4.

Examples 23 to 30, Comparative Examples 7 to 9

Spindle-shaped goethite particles were coated in the same way as inExample 22 by varying the kind and amount of Al, Si, P, Ni, Mg, Co, B orZn compound added.

The main treating conditions are shown in Table 4.

In the Example 23, sodium aluminate was added to the suspension after pHthereof was adjusted to the range of 7 to 9.

In the Example 25, sodium hexametaphosphate and sodium silicate wereadded to the suspension after pH thereof was adjusted to the range of 7to 9.

                  TABLE 4                                                         ______________________________________                                        Example and                                                                            Treated    Compound                                                  Comparative                                                                            particles                  Amount                                    example  (obtained in)                                                                            Kind            (wt %)                                    ______________________________________                                        Example 22                                                                             Example 1  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 13.0                                                          Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             23.2                                                          H.sub.3 BO.sub.3                                                                              15.0                                      Example 23                                                                             Example 10 Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             16.9                                                          Zn(CH.sub.3 COO).sub.2.2H.sub.2 O                                                              1.7                                                          Sodium aluminate                                                                               8.0                                                          H.sub.3 BO.sub.3                                                                              12.0                                      Example 24                                                                             Example 3  Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             12.7                                                          Ni(CH.sub.3 COO).sub.2.4H.sub.2 O                                                              2.8                                                          Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 11.0                                                          H.sub.3 BO.sub.3                                                                              14.0                                      Example 25                                                                             Example 4  Sodium           0.6                                                          hexametaphosphate                                                             Sodium silicate 13.0                                                          Co(NO.sub.3).sub.2.6H.sub.2 O                                                                  8.4                                                          H.sub.3 BO.sub.3                                                                              10.0                                      Example 26                                                                             Example 11 Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 14.0                                                          Co(NO.sub.3).sub.2.6H.sub.2 O                                                                 23.2                                                          H.sub.3 BO.sub.3                                                                              16.0                                      Example 27                                                                             Example 6  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 15.0                                                          Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             21.1                                                          H.sub.3 BO.sub.3                                                                              18.0                                      Example 28                                                                             Example 7  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 16.5                                                          Co(NO.sub.3).sub.2.6H.sub.2 O                                                                 25.3                                                          H.sub.3 BO.sub.3                                                                              20.0                                      Example 29                                                                             Example 8  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 12.0                                                          Mg(CH.sub.3 COO).sub.2.4H.sub.2 O                                                              5.0                                                          H.sub.3 BO.sub.3                                                                              16.0                                      Example 30                                                                             Example 9  Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 12.0                                                          Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                             16.9                                                          H.sub.3 BO.sub.3                                                                              18.0                                      Comparative                                                                            Comparative                                                                              Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 11.0                                      example 7                                                                              example 1  Mg(CH.sub.3 COO).sub.2.4H.sub.2 O                                                              6.0                                                          H.sub.3 BO.sub.3                                                                              14.0                                      Comparative                                                                            Comparative                                                                              Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 11.0                                      example 8                                                                              example 2  Mg(CH.sub.3 COO).sub.2.4H.sub.2 O                                                              6.0                                                          H.sub.3 BO.sub.3                                                                              14.0                                      Comparative                                                                            Comparative                                                                              Al(NO.sub.3).sub.3.9H.sub.2 O                                                                 11.0                                      example 9                                                                              example 3  Mg(CH.sub.3 COO).sub.2.4H.sub.2 O                                                              6.0                                                          H.sub.3 BO.sub.3                                                                              14.0                                      ______________________________________                                    

<Production (I) of Spindle-Shaped Magnetic Iron Based Alloy ParticlesContaining Iron as the Main Ingredient>

Example 31

700 g of the spindle-shaped goethite particles coated with Al, Co and Bcompounds obtained in Example 12 were heat-treated at 410° C. in air toobtain spindle-shaped hematite particles coated with Al, Co and Bcompounds.

100 g of the spindle-shaped hematite particles coated with Al, Co and Bcompounds were charged in about 10 l rotary retort reducing vessel andH₂ gas was passed into the vessel at a rate of 40 l/min while rotatingthe vessel to reduce the hematite particles at a reducing temperature of400° C.

The magnetic iron based alloy particles containing Al, Co and B whichwere obtained by the reduction were taken out while being immersed intoluene liquid so as to prevent from rapid oxidization which may becaused when taken out into air.

A part of the particles were taken out and stable oxide coating filmswere formed on the surfaces while evaporating toluene.

As is obvious form the electron microphotograph (×30000) shown in FIG.7, the magnetic iron based alloy particles containing Al, Co and B werefine particles having an average particle length of 0.27 μm, an aspectratio (major axial diameter/minor axial diameter) of 14.8/1, a specificsurface area of 49.8 m² /g and a crystallite size of 160 Å with auniform particle size distribution and containing no dendrites.

As to the magnetic properties, the coercive force (Hc) was 1,550 Oe andthe saturation magnetization (σs) was 156.9 emu/g.

Examples 32 to 40, Comparative Examples 10 to 12

Magnetic iron based alloy particles were obtained in the same way as inExample 31 except for varying the kind of the starting material, theheat-treating temperature, the kind of the non-reducing atmosphere, thereducing temperature and the flow rate of H₂ gas.

The main producing conditions and various properties of the particlesobtained are shown in Table 5.

Any of the spindle-shaped magnetic iron based alloy particles which wereobtained in Examples 31 to 40 had a uniform particle size distributionwithout containing any dendrites.

The magnetic iron based alloy particles which were obtained in Example35 are shown in an electron micrograph (×30000) of FIG. 8.

                                      TABLE 5                                     __________________________________________________________________________                  Heat treatment in                                                                          Heat treatment                                                                           Properties                                            a non-reducing                                                                             in a reducing                                                                            of the magnetic iron                                  atomospher   gas        based alloy particles                   Example                                                                              Coated        Kind of      Flow     Aspect ratio                       and    particles     the non-     rate of                                                                           Particle                                                                           (major axial                       Comparative                                                                          (obtained                                                                            Temperature                                                                          reducing                                                                            Temperature                                                                          H.sub.2                                                                           length                                                                             diameter/minor                     example                                                                              in)    (°C.)                                                                         atomospher                                                                          (°C.)                                                                         (l/min)                                                                           (μm)                                                                            axial diameter)                    __________________________________________________________________________    Example                                                                              Example                                                                              410    Air   400    40  0.27 14.8/1                             31     12                                                                     Example                                                                              Example                                                                              400    N.sub.2 gas                                                                         380    40  0.30 14.6/1                             32     13                                                                     Example                                                                              Example                                                                              --     --    430    40  0.19 13.0/1                             33     14                                                                     Example                                                                              Example                                                                              530    Air   370    40  0.40 15.3/1                             34     15                                                                     Example                                                                              Example                                                                              430    Air   375    50  0.14  9.2/1                             35     16                                                                     Example                                                                              Example                                                                              380    Air   370    50  0.13  8.4/1                             36     17                                                                     Example                                                                              Example                                                                              400    Air   360    70   0.088                                                                              7.3/1                             37     18                                                                     Example                                                                              Example                                                                              410    Air   380    50  0.24 14.0/1                             38     19                                                                     Example                                                                              Example                                                                              410    Air   390    50  0.30 16.0/1                             39     20                                                                     Example                                                                              Example                                                                              --     --    350    60  0.26 12.0/1                             40     21                                                                     Comparative                                                                          Comparative                                                                          410    Air   400    40  0.20  6.2/1                             example                                                                              example                                                                10     4                                                                      Comparative                                                                          Comparative                                                                          400    Air   400    40  0.23  6.7/1                             example                                                                              example                                                                11     5                                                                      Comparative                                                                          Comparative                                                                          400    Air   400    40  0.25 10.1/1                             example                                                                              example                                                                12     6                                                                      __________________________________________________________________________                               Properties of the magnetic                                                    iron based alloy particles                                      Example                                                                              Coated       Specific  Saturation                                      and    particles    surface                                                                            Coercive                                                                           magnetization                                   Comparative                                                                          (obtained                                                                            Crystallite                                                                         area force                                                                              (σs)                                      example                                                                              in)    Size (Å)                                                                        (m.sup.2 /g)                                                                       (Oe) (emu/g)                            __________________________________________________________________________                 Example                                                                              Example                                                                              160   49.8 1550 156.9                                           31     12                                                                     Example                                                                              Example                                                                              155   48.7 1530 159.1                                           32     13                                                                     Example                                                                              Example                                                                              172   51.3 1625 158.7                                           33     14                                                                     Example                                                                              Example                                                                              153   47.8 1465 157.3                                           34     15                                                                     Example                                                                              Example                                                                              148   52.4 1590 159.5                                           35     16                                                                     Example                                                                              Example                                                                              140   55.4 1526 158.4                                           36     17                                                                     Example                                                                              Example                                                                              135   58.7 1500 157.6                                           37     18                                                                     Example                                                                              Example                                                                              150   50.4 1638 157.6                                           38     19                                                                     Example                                                                              Example                                                                              147   52.0 1650 158.4                                           39     20                                                                     Example                                                                              Example                                                                              160   45.8 1445 145.3                                           40     21                                                                     Comparative                                                                          Comparative                                                                          210   47.0 1050 158.3                                           example                                                                              example                                                                10     4                                                                      Comparative                                                                          Comparative                                                                          190   67.3 1350 159.6                                           example                                                                              example                                                                11     5                                                                      Comparative                                                                          Comparative                                                                          188   60.3 1530 156.8                                           example                                                                              example                                                                12     6                                                         __________________________________________________________________________

<Production (II) of Spindle-Shaped Magnetic Iron Based Alloy Particles>

Example 41

5.0 kg of the spindle-shaped goethite particles coated with Al, Co and Bcompounds obtained in Example 22 were heat-treated at 400° C. in air toobtain spindle-shaped hematite particles coated with Al, Co and Bcompounds.

2000 g of the spindle-shaped hematite particles coated with Al, Co and Bcompounds were changed in a fluidized bed reducing furnace, and H₂ gaswas passed in to the furnace at a rate of 180 l/min to reduce thehematite particles at a reducing temperature of 390° C. for 15 hours.

After the reduction, H₂ gas was replaced by N₂ gas and the particleswere cooled to 50° C. while blowing N₂ gas thereinto at a rate of 160Nl/min. While maintaining the temperature of the furnace at 50° C., N₂gas and air which were mixed in the ratio of 160N l/min to 0.2N l/min inthe flow rate were passed into the furnace. When it was observed thatthe heat generation by the mixed gas having the above mixing ratio hadreached the peak, the flow rate of air was raised to 0.4N l/min so as toincrease the air ratio in the mixed gas. In this way, the air ratio inthe mixed gas was sequentially raised by raising it when the peak of theheat generation by the oxidization was observed and finally oxidizationwas continued with air and N₂ gas mixed in the ratio of 1.2N l/min to160N l/min in the flow rate until no heat was produced by theoxidization and the temperature of the particles became about 50° C.which was almost the same as the temperature of the furnace. During thisprocess, the temperature of the particles had reached 75° C. at thehighest.

The air mixing ratio was then gradually raised while maintaining thetemperature of the furnace at 50° C. and the flow rate of the N₂ gas at160N l/min until the flow rate of air was 20N l/min. During thisprocess, no heat was observed.

While further passing air and N₂ gas mixed in the ratio of 40N l/min to140N l/min in the flow rate into the furnace, the temperature of theparticles was cooled to room temperature.

The mixed gas was then replaced by N₂ gas by reducing the air flow rateto 0 l/min and the thus-obtained spindle-shaped magnetic iron basedalloy particles with surface coating films formed on the surfaces werecollected.

As is obvious from the electron micrograph (×30000) shown in FIG. 9, themagnetic iron based alloy particles containing Al, Co and B were fineparticles having an average particle length of 0.28 μm, an aspect ratio(major axial diameter/minor axial diameter) of 15.0/1, a specificsurface area of 49.2 m² /g and a crystallite size of 155 Å with auniform particle size distribution and containing no dendrites.

As to the magnetic properties, the coercive force (Hc) was 1,530 Oe, thesaturation magnetization (σs) was 135.4 emu/g and the reduction ratio ofsaturation magnetization was 4.2%.

Examples 42 to 50, Comparative Examples 13 to 15

Magnetic iron based alloy particles were obtained in the same way as inExample 41 except for varying the kind of the starting material, theheat-treatment temperature, the kind of the non-reducing atmosphere, thereducing temperature, the flow rate of H₂ and the surface oxidizingconditions.

The main producing conditions and various properties of the particlesobtained are shown in Table 6.

Any of the spindle-shaped magnetic iron based alloy particles which wereobtained in Examples 42 to 50 had a uniform particle size distributionwithout containing any dendrites.

The magnetic iron based alloy particles which were obtained in Example46 are shown in an electron micrograph (×30000) of FIG. 10.

The magnetic iron based alloy particles obtained after reduction inComparative Examples 13 to 15 were taken out while being immersed intoluene liquid so as to prevent rapid oxidization which may be causedwhen taken out into air.

A part of the particles were taken out for measurement and stable oxidecoating films were formed on the surfaces while evaporating toluene.

                                      TABLE 6                                     __________________________________________________________________________                                            Heat treatment in an                                Heat treatment in                                                                          Heat treatment                                                                             oxygen-containing                                   a non-reducing                                                                             in a reducing                                                                              gas                                                 atomospher   gas                          Ratio                 Example                                                                              Coated        Kind of       Flow Temperature     (air/N.sub.2)         and    particles     non-          rate of                                                                            of the  Maximum at the                Comparative                                                                          (obtained                                                                            Temperature                                                                          reducing                                                                            Temperature                                                                           H.sub.2                                                                            furnace temperature                                                                           maximum               example                                                                              in)    (°C.)                                                                         atomospher                                                                          (°C.)                                                                          (l/min)                                                                            (°C.)                                                                          (°C.)                                                                          temperture            __________________________________________________________________________    Example                                                                              Example                                                                              400    Air   390     180  50      75      1.2/160               41     22                                                                     Example                                                                              Example                                                                              400    N.sub.2 gas                                                                         400     180  80      160     4.0/160               42     23                                                                     Example                                                                              Example                                                                              --     --    380     180  30      54      0.8/160               43     24                                                                     Example                                                                              Example                                                                              550    Air   400     200  80      105     0.8/160               44     25                                                                     Example                                                                              Example                                                                              --     --    360     200  40      72      1.6/160               45     26                                                                     Example                                                                              Example                                                                              380    Air   370     200  40      74      1.6/160               46     27                                                                     Example                                                                              Example                                                                              400    Air   360     210  40      81      2.0/160               47     28                                                                     Example                                                                              Example                                                                              400    Air   430     200  50      73      1.2/160               48     29                                                                     Example                                                                              Example                                                                              410    Air   400     200  50      82      1.6/160               49     30                                                                     Example                                                                              Example                                                                              400    Air   370     200  60      83      1.2/160               50     26                                                                     Comparative                                                                          Comparative                                                                          410    Air   410     180  50      78      1.2/160               example                                                                              example 7                                                              13                                                                            Comparative                                                                          Comparative                                                                          400    Air   410     180  50      75      1.2/160               example                                                                              example 8                                                              14                                                                            Comparative                                                                          Comparative                                                                          400    Air   410     180  50      75      1.2/160               example                                                                              example 9                                                              15                                                                            __________________________________________________________________________                         Properties of the magnetic iron based alloy                                   particles                                                                          Aspect ratio                  Reduction                    Example                                                                              Coated      (major axial Specific  Saturation                                                                           ratio of                     and    particles                                                                            Particle                                                                           diameter/    surface                                                                            Coercive                                                                           magnetization                                                                        saturation                   Comparative                                                                          (obtained                                                                            length                                                                             minor axial                                                                          Crystallite                                                                         area force                                                                              σs                                                                             magnetization                example                                                                              in)    (μm)                                                                            diameter)                                                                            Size (Å)                                                                        (m.sup.2 /g)                                                                       (Oe) (emu/g)                                                                              (%)                   __________________________________________________________________________           Example                                                                              Example                                                                              0.28 15.0/1 155   49.2 1530 135.4  4.2                          41     22                                                                     Example                                                                              Example                                                                              0.30 14.0/1 160   47.8 1515 117.3  0.5                          42     23                                                                     Example                                                                              Example                                                                              0.19 13.2/1 158   52.1 1620 139.5  4.9                          43     24                                                                     Example                                                                              Example                                                                              0.37 15.0/1 170   45.3 1500 128.2  2.6                          44     25                                                                     Example                                                                              Example                                                                              0.14  9.5/1 145   53.1 1570 136.2  4.5                          45     26                                                                     Example                                                                              Example                                                                              0.13  8.5/1 138   54.7 1530 129.4  4.2                          46     27                                                                     Example                                                                              Example                                                                               0.085                                                                              7.5/1 130   59.0 1500 125.6  4.5                          47     28                                                                     Example                                                                              Example                                                                              0.25 14.5/1 165   48.2 1620 136.8  3.3                          48     29                                                                     Example                                                                              Example                                                                              0.31 16.2/1 143   51.5 1635 132.4  3.6                          49     30                                                                     Example                                                                              Example                                                                              0.14  9.0/1 145   53.4 1585 137.1  4.5                          50     26                                                                     Comparative                                                                          Comparative                                                                          0.21  6.0/1 215   46.8 1040 155.6  17.3                         example                                                                              example 7                                                              13                                                                            Comparative                                                                          Comparative                                                                          0.23  6.5/1 190   66.8 1335 154.2  26.2                         example                                                                              example 8                                                              14                                                                            Comparative                                                                          Comparative                                                                          0.25 10.0/1 185   59.6 1510 157.5  22.3                         example                                                                              example 9                                                              15                                                                     __________________________________________________________________________

What is claimed is:
 1. A process for producing spindle-shaped magneticiron based alloy particles comprising the steps of:(1) adding an aqueousalkali carbonate solution or a mixture of an aqueous alkali carbonatesolution and an aqueous alkali hydroxide solution to an aqueous ferroussalt solution so as to obtain a suspension containing FeCO₃ or anFe-containing precipitate; (2) aging the thus-obtained suspensioncontaining FeCO₃ or an Fe-containing precipitate at 35° to 60° C. for 50to 800 minutes at a pH of 7 to 11 in a non-oxidizing atmosphere; (3)oxidizing the suspension by passing an oxygen-containing gas into theaged suspension containing FeCO₃ or an Fe-containing precipitate in thepresence of 0.1 to 10.0 mol % of propionic acid or a salt thereof basedon Fe at 35° to 70° C. so as to obtain spindle-shaped goethiteparticles; (4) coating the thus-obtained spindle-shaped goethiteparticles with at least one compound selected from the group consistingof Ni, Al, Si, P, Co, Mg, B and Zn compounds; and (5) heat-treating thecoated spindle-shaped particles in a reducing gas to produce reducedparticles.
 2. A process according to claim 1, further comprising thestep of heating and dehydrating said spindle-shaped goethite particlesto produce spindle-shaped hematite particles before the coating step. 3.A process according to claim 1, further comprising the step ofheat-treating said coated particles at 300° to 800° C. in a non-reducingatmosphere before the heat-treating step.
 4. A process according toclaim 1, further comprising the step of (6) oxidizing the surfaces ofsaid reduced particles prepared in step (5) in an oxygen-containing gas.5. A process according to claim 4, wherein the surfaces of said reducedparticles are oxidized at 30° to 200° C. in an oxygen-containing gashaving an oxygen content of 0.02 to 20 vol %.
 6. A process according toclaims 1, wherein said propionic acid or a salt thereof is added to saidaqueous alkali carbonate solution, said mixture of said aqueous alkalicarbonate solution and said aqueous alkali hydroxide solution, saidferrous salt solution, or said suspension containing FeCO₃ or anFe-containing precipitate before the oxidization.
 7. A process forproducing spindle-shaped goethite particles comprising the steps of:adding an aqueous alkali carbonate solution or a mixture of an aqueousalkali carbonate solution and an aqueous alkali hydroxide solution to anaqueous ferrous salt solution so as to obtain a suspension containingFeCO₃ or an Fe-containing precipitate; aging the thus-obtainedsuspension containing FeCO₃ or an Fe-containing precipitate at 35° to60° C. for 50 to 800 minutes at pH of 7 to 11 in a non-oxidizingatmosphere; and carrying out oxidization by passing an oxygen-containinggas into the aged suspension containing FeCO₃ or an Fe-containingprecipitate in the presence of 0.1 to 10.0 mol l % of propionic acid ora salt thereof based on Fe at 35° to 70° C.
 8. A process according toclaim 7, wherein said propionic acid or a salt thereof is added to saidaqueous alkali carbonate solution, said mixture of said aqueous alkalicarbonate solution and said aqueous alkali hydroxide solution, saidferrous salt solution or said suspension containing FeCO₃ or anFe-containing precipitate before the oxidization.